A recent television broadcasting system, for example, can broadcast a number of program contents to viewers, as multi-channel broadcasting is developed along with a widespread use of digital broadcast. Also, the number of services for distributing video contents has been increasing along with a widespread use of the high-speed internet in the internet environment.
Adjunct data (for example, data broadcast in the digital broadcast) constituted of data signals are sometimes added to contents distributed by such a digital broadcasting system or a contents distribution service of the internet or the like. The adjunct data may include, for example, a script to execute an application (also referred simply to “Appri” hereinafter) such as e-mail or to jump to an external destination linked by a URL (Uniform Resource Locator: Internet Address).
Thus, with regard to the contents distribution service, it is necessary to ensure a security of the contents to a viewer. This is because it may happen that, for example, in contents distributed by contents distribution service provider, a legitimate contents stream being received is switched to an unintended stream by a malicious assailant, so that a user receives a fraudulent stream including a packet whose relationship with its previous or following packet is not guaranteed.
As for the digital broadcast, the reception apparatus may receive a fraudulent stream to which so-called “spoofing” is performed, with a falsified data broadcast in which interactive communication is possible, and image data and audio data both of which remain intact. In such a case, there is a concern that a user executes an application included in the data broadcast or a script to jump to an external destination believing that information should be safe as it is distributed by a trustworthy broadcast station, which may result in damaging the user seriously by phishing, viewing or recording programs tampered, undesired data written in a nonvolatile memory, and the likes.
Such problems may happen more often as a widespread of broadcasting type such as a contents distribution system (a so-called “Spot-cast”) using weak radio waves, in particular, which is recently suggested and utilizes One-segment broadcasting of Digital Terrestrial Television.
Hence, in the contents distribution service, it is necessary to ensure the security of the contents. As one method, there is known a stream authentication which ensures legitimacy of a stream by adding a digital signature for authenticating a sender identification to the stream of contents transmitted by the sender and authenticating the sender by decrypting the digital signature with a public key of the sender (see Patent Document 1 and Patent Document 2, for example).
As described above, if a transmission apparatus transmits a stream of contents whose security is ensured by the stream authentication, the reception apparatus may refrain from playing the stream received until it is ensured to be legitimate (that is, during a process of authentication), which may prevent inflicting a disadvantage on the user.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-249932    Patent Document 2: Japanese Patent Application Laid-Open No. 2001-251296